1. Field of the Invention
The present invention relates to a wafer processing method of dividing a wafer along a plurality of crossing streets formed on the front side of the wafer to thereby partition a plurality of regions where a plurality of devices are respectively formed along the streets, thus obtaining the individual devices.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby partition a plurality of regions where a plurality of devices such as ICs and LSIs are respectively formed. The semiconductor wafer is cut along the streets to thereby divide the regions where the devices are formed from each other, thus obtaining the individual devices. Further, an optical device wafer is provided by forming a gallium nitride compound semiconductor layer or the like on the front side of a sapphire substrate or a silicon carbide substrate. The optical device wafer is also cut along the streets to obtain individual optical devices divided from each other, such as light emitting diodes and laser diodes, which are widely used in electric equipment.
As a method of dividing such a wafer along the streets, there has been tried a laser processing method of applying a pulsed laser beam having a transmission wavelength to the wafer along the streets in the condition where the focal point of the pulsed laser beam is set inside the wafer in a subject area to be divided. More specifically, this wafer dividing method using laser processing includes the steps of applying a pulsed laser beam having a transmission wavelength to the wafer from one side of the wafer along the streets in the condition where the focal point of the pulsed laser beam is set inside the wafer to thereby continuously form a modified layer inside the wafer along each street and next applying an external force to the wafer along each street where the modified layer is formed to be reduced in strength, thereby dividing the wafer into the individual devices (see Japanese Patent No. 3408805, for example).
According to the wafer dividing method described in Japanese Patent No. 3408805, however, the modified layer is left on the side surface of each device after dividing the wafer, causing a problem such that the die strength of each device is reduced to cause a degradation in quality of each device. To solve this problem, there has been proposed a wafer processing method including the steps of applying a laser beam having a transmission wavelength to the wafer along the streets to thereby form a modified layer having a predetermined thickness inside the wafer along each street at a predetermined depth from the back side of the wafer, next dividing the wafer along each street where the modified layer is formed, and finally grinding the back side of the wafer to remove the modified layer (see Japanese Patent Laid-Open No. 2005-86161, for example).